The radio communication technology is faced by a problem of a lack of frequency resources due to recent rapid increase in data traffic. One of examples of major solutions to use frequency resources efficiently is to introduce a relay station. A relay station relays traffic for a terminal located inside a cell or in the vicinity thereof where radio waves are weak (e.g., behind a building, inside a building, or near a cell edge). Another example of major solutions to use frequency resources efficiently is to introduce a small cell. A small cell is a concept encompassing femtocells, nanocells, picocells, microcells, and the like. A station of a small cell is installed at a position where radio waves are weak or the traffic amount is large and operates a radio communication system of a small cell.
To introduce a small cell, it is required that a macrocell is not interfered harmfully. One of techniques to prevent interference with a macrocell is to divide frequencies. That is, a base station of a small cell uses a frequency channel that is different from a frequency channel used in s macrocell, and thereby it becomes possible to suppress an interference level given by the base station of a small cell or a terminal thereof to a base station of a macrocell or a terminal thereof.
Patent Literature 1 below proposes a technique in which a base station of a picocell secondarily uses a part of a frequency channel protected for a macrocell in a situation that the macrocell is operated with a frequency division duplex (FDD) scheme. The base station of the picocell operates a radio communication system of a small cell with a time division duplex (TDD) scheme.